In classical black-and-white photography a photographic element containing a silver halide emulsion layer coated on a transparent film support is imagewise exposed to light. This produces a latent image within the emulsion layer. The film is then photographically processed to transform the latent image into a silver image that is a negative image of the subject photographed. Photographic processing involves developing (reducing silver halide grains containing latent image sites to silver), stopping development, and fixing (dissolving undeveloped silver halide grains). The resulting processed photographic element, commonly referred to as a negative, is placed between a uniform exposure light source and a second photographic element, commonly referred to as a photographic paper, containing a silver halide emulsion layer coated on a white paper support. Exposure of the emulsion layer of the photographic paper through the negative produces a latent image in the photographic paper that is a positive image of the subject originally photographed. Photographic processing of the photographic paper produces a positive silver image. The image bearing photographic paper is commonly referred to as a print.
In a well known, but much less common, variant of classical black-and-white photography a direct positive emulsion can be employed, so named because the first image produced on processing is a positive silver image, obviating any necessity of printing to obtain a viewable positive image. Another well known variation, commonly referred to as instant photography, involves imagewise transfer of silver ion to a physical development site in a receiver to produce a viewable transferred silver image.
In classical color photography the photographic film contains three superimposed silver halide emulsion layer units, one for forming a latent image corresponding to blue light (i.e., blue) exposure, one for forming a latent image corresponding to green exposure and one for forming a latent image corresponding to red exposure. During photographic processing developing agent oxidized upon reduction of latent image containing grains reacts to produce a dye image with silver being an unused product of the oxidation-reduction development reaction. Developed silver (Ag.degree.) is removed by bleaching during photographic processing. The image dyes are complementary subtractive primaries--that is, yellow, magenta and cyan dye images are formed in the blue, green and red recording emulsion layers, respectively. This produces negative dye images (i.e., blue, green and red subject features appear yellow, magenta and cyan, respectively). Exposure of color paper through the color negative followed by photographic processing produces a positive color print. Again, bleaching removes developed silver that would otherwise blacken the color print.
In one common variation of classical color photography reversal processing is undertaken to produce a positive dye image in the color film(commonly referred to as a slide, the image typically being viewed by projection). In another common variation, referred to as color image transfer or instant photography, image dyes are transferred to a receiver for viewing.
In each of the classical forms of photography noted above the final image is intended to be viewed by the human eye. Thus, the conformation of the viewed image to the subject image, absent intended aesthetic departures, is the criterion of photographic success.
With the emergence of computer controlled data processing capabilities, interest has developed in extracting the information contained in an imagewise exposed photographic element instead of proceeding directly to a viewable image. It is now common practice to extract the information contained in both black-and-white and color images by scanning. The most common approach to scanning a black-and-white negative is to record point-by-point or line-by-line the transmission of a near infrared beam, relying on developed silver to modulate the beam. In color photography blue, green and red scanning beams are modulated by the yellow, magenta and cyan image dyes. In a variant color scanning approach the blue, green and red scanning beams are combined into a single white scanning beam modulated by the image dyes that is read through red, green and blue filters to create three separate records. The records produced by image dye modulation can then be read into any convenient memory medium (e.g., an optical disk). The advantage of reading an image into memory is that the information is now in a form that is free of the classical restraints of photographic embodiments. For example, age degradation of the photographic image can be for all practical purposes eliminated. Systematic manipulation (e.g., image reversal, hue alteration, etc.) of the image information that would be cumbersome or impossible to achieve in a controlled and reversible manner in a photographic element are readily achieved. The stored information can be retrieved from memory to modulate light exposures necessary to recreate the image as a photographic negative, slide or print at will. Alternatively, the image can be viewed as a video display or printed by a variety of techniques beyond the bounds of classical photography--e.g., xerography, ink jet printing, dye diffusion printing, etc.
Hunt U.K. 1,458,370 illustrates a color photographic element constructed to have three separate color records extracted by scanning. Hunt employs a classical color film modified by the substitution of a panchromatic sensitized silver halide emulsion layer for the green recording emulsion layer. Following imagewise exposure and processing three separate records are present in the film, a yellow dye image recording blue exposure, a cyan dye image recording red exposure and a magenta dye image recording exposure throughout the visible spectrum. These three dye images are then used to derive blue, green and red exposure records, but the photographic element itself is not properly balanced to be used as a color negative is classically used for photographic print formation.
A number of other unusual film constructions have been suggested for producing photographic images intended to be extracted by scanning:
Kellogg et al U.S. Pat. No. 4,788,131 extracts image information from an imagewise exposed photographic element by stimulated emission from latent image sites of photographic elements held at extremely low temperatures. The required low temperatures are, of course, a deterrent to adopting this approach.
Levine U.S. Pat. No. 4,777,102 relies on the differential between accumulated incident and transmitted light during scanning to measure the light unsaturation remaining in silver halide grains after exposure. This approach is unattractive, since the difference in light unsaturation between a silver halide grain that has not been exposed and one that contains a latent image may be as low as four photons and variations in grain saturation can vary over a very large range.
Schumann et al U.S. Pat. No. 4,543,308 relies upon differentials in luminescence in developed and fixed color films to provide an image during scanning. Relying on differentials in luminescence from spectral sensitizing dye, the preferred embodiment of Schumann et al, is unattractive, since luminescence intensities are limited. Increasing spectral sensitizing dye concentrations beyond optimum levels is well recognized to desensitize silver halide emulsions.
Unusual silver halide photographic element constructions for producing images intended to be extracted by scanning have employed the same silver halide emulsions developed for classical black-and-white and color photography. The silver halide grain population of an emulsion can take a wide variety of forms. The silver halide grains themselves can take varied shapes. Regular grains, those free of internal stacking faults or screw dislocations, are typically cubes or octahedra, although rhombododecahedra and four additional rarely encountered regular geometric forms are known. Cubes are bounded entirely by {100} crystal faces; octahedra are bounded entirely by {111} crystal faces; and rhombododecahedra are bounded entirely by {110} crystal faces. There are a variety of grain structures that exhibit a combination of crystal faces lying in different crystal planes--e.g., tetradecahedra (a.k.a. cubo-octahedra) have six {100} crystal faces and eight {111} crystal faces. Emulsions prepared in an active ripening environment, such as ammoniacal emulsions, often have had the grain corners sufficiently rounded that the grains are essentially spherical. Many, if not most, silver halide grains found in photographic emulsions are not regular. Twinning is a common grain irregularity. Singly twinned grains are common. Tabular grains having {111} major faces are produced by two or three parallel twin planes. Multiply twinned grains are often of irregular shape and have on at least one occasion been descriptively referred to as "potato" grains. To complicate matters further, silver halide emulsions usually contain a mixture of grains of different sizes and shapes. Thus, nominal references to photographic silver halide emulsions embrace a large variety of silver halide grain populations.
Although tabular grains had been observed in silver bromide and bromoiodide photographic emulsions dating from the earliest observations of magnified grains and grain replicas, it was not until the early 1980's that photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of blue and minus blue imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, were realized to be attainable from "tabular grain" silver bromide and bromoiodide emulsions in which the majority (&gt;50%) of the total grain population based on grain projected area is accounted for by tabular grains satisfying the mean tabularity relationship: EQU ECD/t.sup.2 &gt;25
where
ECD is the equivalent circular diameter in micrometers (.mu.m) of the tabular grains and
t is the thickness in .mu.m of the tabular grains. Once photographic advantages were demonstrated with tabular grain silver bromide and bromoiodide emulsions techniques were devised to prepare tabular grains containing silver chloride alone or in combination with other silver halides. Subsequent investigators have extended the definition of tabular grain emulsions to those in which the mean aspect ratio (ECD:t) of grains having parallel crystal faces is as low as 2:1. Photographic advantages attributable to the tabular grain shape can be realized with tabularities of greater than 8.
Although most tabular grain emulsion definitions require greater than 50 percent of the total grain projected area to be accounted for by tabular grains, tabular grain emulsions often contain significant unwanted grain populations and also exhibit a higher level of grain dispersity (ECD variance) than can be obtained by a well controlled precipitation of a regular grain emulsion. This has presented a continuing challenge to those preparing tabular grain emulsions.
A statistical technique for quantifying tabular grain dispersity that has been applied to both nontabular and tabular grain emulsions is to obtain a statistically significant sampling of the individual tabular grain projected areas, calculate the corresponding ECD of each grain, determine the standard deviation of the grain ECDs, divide the standard deviation of the grain population by the mean ECD of the grains sampled and multiply by 100 to obtain the size coefficient of variation, hereinafter referred to as COV(ECD), of the grain population as a percentage.
Kofron et al U.S. Pat. No. 4,439,520 illustrates tabular grain emulsion technology at the outset of its development in the early 1980's and multicolor photographic elements containing these emulsions.
Saitou et al U.S. Pat. No. 4,797,354 reports in its Examples tabular grain silver bromide emulsions with tabular grain projected areas of up to 93 percent. Of the tabular grain emulsions having a tabular grain projected area of greater than 90%, a mean ECD of at least 0.4 .mu.m and a mean thickness of less than 0.2 .mu.m the lowest COV(ECD) reported is 15.3%.
Nakamura et al U.S. Pat. No. 5,096,806 reports in Example 2 a tabular grain silver bromoiodide emulsion having a tabular grain projected area of 99.7%, a mean tabular grain thickness of 0.16 .mu.m, a mean ECD of 1.1 .mu.m. and grain COV(ECD) of 10.1%.
Tsaur et al U.S. Pat. Nos. 5,147,771, 5,147,772, 5,147,773 and 5,171,659 disclose processes of preparing tabular grains silver bromide and bromoiodide emulsions employing varied polyalkylene oxide block copolymer to reduce grain dispersity. Although no quantification is provided, Tsaur et al U.S. Pat. No. 5,147,771 and 5,171,659 are notable in observing qualitatively the reduced thickness variance of one of the tabular grain emulsions prepared.
Buhr et al Research Disclosure, Vol. 253, May 1985, Item 25330, presents in FIG. 1 a calculated correlation between sheet thicknesses of from 0.07 .mu.m to 0.16 .mu.m and reflectances at varied visible wavelengths. Based on the calculated reflectances of thin sheets Buhr et al suggests employing tabular grain emulsions for varied layers of a multicolor photographic element to minimize reflectance of light intended to be recorded by underlying emulsion layers or to maximize reflectance of blue light before it can reach one or more underlying emulsion layers and thereby contaminate a minus blue (green or red) image record.